Shock absorbers of the hydraulic type provided with devices capable of damping the vibrations of the road surface, which otherwise would be discharged onto the arms and back of the driver, making driving tiring and wearying, are currently known and adopted successfully in many types of motorcycles, for example for track racing.
The operation of these hydraulic shock absorbers is based on the principle of the bleeding of a viscous fluid such as oil between the two chambers formed inside a tubular body that is closed at its end by a piston moved by a stem that exits from the tubular body and is to be associated with a frame or a wheel.
One of these two chambers is termed high-pressure chamber and the other one is termed low-pressure chamber.
The motion of the piston causes the bleeding of fluid through the calibrated orifices of the piston from the high-pressure chamber toward the low-pressure chamber.
In known types of hydraulic shock absorbers, a compensation chamber is generally associated with the high-pressure chamber, which is designed to make space for the oil displaced by the entry of the stem in the tubular body, and is often formed in a cup which is external to the main body of the shock absorber, sometimes with the interposition of valve means for adjusting the oil flows, such as a two-way valve or the like.
This external cup is constituted typically by a tubular body which is closed hermetically by end covers and in which a partition is accommodated which can slide or is elastically deformable and forms two chambers, in one of which the incompressible fluid that arrives from the pressurized chamber of the shock absorber is bled and in the second one of such chambers of the cup there is compressible fluid, which defines the means for the elastic return in extension of the shock absorber.
Such external cup is designed to absorb the volume variations inside the tubular body caused by the entry of the piston stem as well as the volume variations of the incompressible fluid due to its heating, and to make the transition from the compression step to the extension step of the shock absorber less abrupt and at the same time not excessively delayed.
Shock absorbers having a structure substantially like the one described above are currently used effectively, yet are not free from drawbacks.
A first one of these drawbacks is the production of high pressure in the elastic return means with compressible fluid in the external cup, which, as mentioned, are needed in order to absorb the volume of oil that cannot remain in the high-pressure chamber, but do not contrast effectively the high pressure transmitted by the associated high-pressure chamber; accordingly, the force that one wishes to generate with the motion of the piston is proportional not only to the resistance that the orifices for the passage of the incompressible fluid offer to the flow of the fluid from the high-pressure chamber to the low-pressure chamber, but also to the elastic response of the elastic return means with compressible fluid that are present in the cup.
This causes a certain delay in the response of the piston in the transition from a compression step to an extension step of the shock absorber or vice versa, a delay which is obviously reflected in the lack of correct trim of the motorcycle when the next stress occurs, and therefore affects the rider.
A second drawback consists in that an external cup, with or without an interposed valve, entails the production and installation of a certain number of components, with an increase both in terms of weight of the fork of the motorcycle and in terms of production, assembly and maintenance costs.
In particular, as regards the weight of the fork, such fork is a non-suspended mass, the inertia of which affects the cycling balance of the motorcycle; moreover, the reciprocating motion of the wheel, rigidly connected to the lower end of the fork, is greatly affected, especially at high frequencies, by the inertia of the external elements that are connected to the sleeve of the fork.
These drawbacks described so far for a motorcycle can occur in the same manner with the shock-absorbing means of a car or of another vehicle in general, as well as with the shock-absorbing means of equipment, machine tools and machinery in general that generate vibrations and stresses which are continuous but have a variable frequency and intensity.